1. Field of the Invention
The present invention relates to a rangefinder device for measuring a distance to a target.
2. Description of the Related Art
Rangefinders are used to measure distances to targets. Typically, a rangefinder will emit a beam of energy towards a target and detect a reflected beam from the target. In general, the rangefinder measures the time interval between the emission of the transmitted beam and the reception of the reflected beam. This time interval is referred to herein as the “flight time.” The distance from the rangefinder to the target is derived from the speed of the beam and the flight time.
It can be difficult for conventional receivers to detect a reflected beam because the strength of the reflected beam becomes weaker as a function of distance to the target. Further, factors such as rain, snow, shrubbery, targets with poor reflectivity, or the like can reduce the strength of the reflected beam.
The ability of a rangefinder to detect a reflected beam from a target depends at least in part on the sensitivity of the rangefinder's receiver. In general, the magnitude of the received signal should be larger than the noise present in the receiver in order to be detected. Receiver noise is typically generated by, for example, a photodiode used to detect the reflected beam, receiver electronics used to amplify a signal from the photodiode, background light received along with the reflected beam, or the like. Background light is typically the largest source of noise during daylight operation of laser rangefinders. Such noise reduces the signal-to-noise ratio and makes it difficult to detect weak reflections.
While detection depends at least in part on the receiver's sensitivity, a receiver that is too sensitive can generate a false detection of the reflected beam. False detection may be caused, for example, by reflections from objects that are not the intended target such as shrubs, trees, snow, rain, or other objects in the path between the intended target and the rangefinder. Weak reflections from such objects can cause an overly sensitive receiver to incorrectly determine that a beam reflected from the intended target has been received. However, as discussed above, if the sensitivity is too low, valid reflections from distant targets or targets with poor reflectivity cannot be detected.
The accuracy of range measurements is affected by the rangefinder's ability to accurately measure a beam's flight time because small variations can create significant errors in the distance calculated. Often, capacitor discharge mechanisms are used to create a more manageable representation of the flight time. Even using such discharging mechanisms, delays in the rangefinder's internal circuitry add additional error to flight time interval measurements.
Some errors caused by the internal circuitry are due to inherent delays that can be initially measured and corrected during, for example, factory calibration and test. However, some delays caused by the internal circuitry are variable and may change over time. Further, some delays may change with variations in environmental conditions such as temperature, humidity and the like.